The present invention relates to a novel hydroconversion catalyst for heavy petroleum fractions, comprising as basic constituent at least one acid zeolite of the specially modified Y type.
The hydrocracking or cracking of heavy petroleum fractions are very important processes of refining which enable the production from surplus, low value heavy charges of lighter fractions such as light distillates and fuels, jet fuels and light gas oils which are sought by the refiner to adapt his production to the structure of the market. With respect to catalytic cracking, the advantage of catalytic hydrocracking is to provide middle distillates, jet fuels and gas oils of very good quality. On the other hand, the gasoline produced has a much lower octane number than that derived from catalytic cracking.
The invention relates to a novel catalyst containing particularly (a) a matrix and (b) a particular zeolite, this catalyst being utilizable either in hydrocracking reactions or in cracking reactions.
The catalysts used in hydrocracking are all of the bifunctional type associating an acid function with a hydrogenating function. The acid function is contributed by supports of large surface areas (150 to 800 m.sup.2.g.sup.-1 approximately) having a surface acidity, such as halogenated aluminas (chlorinated or fluorinated particularly), combinations of boron and aluminum oxides, amorphous silica-aluminas and zeolites. The hydrogenating function is contributed either by one or several metals of group VIII of the periodic classification of the elements, such as nickel, palladium or platinum for example, or by an association of at least two metals selected from group VI of the periodic classification, molybdenum and tungsten particularly, and VIII of the same classification, cobalt and nickel particularly, two at least of the metals of this association belonging to two different groups (VI and VIII previously mentioned).
The balance between the two acid and hydrogenating functions is a fundamental parameter which governs the activity and selectivity of the catalyst. A weak acid function and a stronger hydrogenating function give catalysts of little activity, operating generally at high temperature (390.degree. C. approximately), and at a low volumetric feed rate (VVH expressed by volume of charge processed per unit volume of catalyst and per hour is generally less than or equal to 2, but endowed with very good selectivity of middle distillates. Conversely, strong acid function and weak hydrogenating function give very reactive catalysts but which have poor selectivity of middle distillates.
Thus, it is one of the great advantages of hydrocracking to show great flexibility at various levels: flexibility, at the level of the types of catalyst used, which results in flexibility at the level of the charges processed and at the level of the products obtained. An easy parameter to control is the acidity of the support of the catalyst.
In supports of little acidity, are found the family of amorphous silica-aluminas. Many catalysts on the hydrocracking market are constituted by silica-alumina associated, either with a metal of group VIII, or preferably when the content of heteroatomic poisons of the charge to be processed exceed 0.5% by weight, with an association of sulphides of the metals of groups VI B and VIII. These systems have very good selectivity of middle distillates, and the products formed are of good quality. These catalysts, for the less acid among them, may also produce lubricant bases. The drawback of all these catalytic systems based on an amorphous support, is, as has been stated, their low activity.
Acid zeolites present the advantage with respect to the other previously mentioned acid supports of contributing a much higher acidity. The new catalysts which contain them are therefore much more active and for this reason, enable operation at a lower temperature and/or at a higher volumetric feed rate (VFR). On the other hand, this higher acidity modifies the equilibrium between the two acid and hydrogenating catalytic functions. There results therefrom a notable modification of selectivity of these catalysts with respect to conventional catalysts: they have more cracking ability and produce consequently much more gasoline than middle distillates.
The present invention relates to a novel type of catalyst containing 2 to 80% by weight, preferably 3 to 50% of a zeolite whose physical characteristics and acidity have been specially modified, and which has an activity and selectivity of middle distillates particularly improved with respect to other systems of the prior art based on zeolites.
The zeolite used in the catalyst of the present invention is an HY acid zeolite characterised by various specifications, of which the methods of determination will be specified in description below; a molar ratio SiO.sub.2 /Al.sub.2 O.sub.3 comprised between 8 and 70 and preferably between about 12 and 40; a sodium content less than 0.5% by weight determined on the zeolite calcined at 1100.degree. C.; a crystalline parameter a with an elementary mesh comprised between 24.55.times.10.sup.-10 m and 24.24.times.10.sup.-10 m and preferably between 24.38.times.10.sup.-10 m and 24.26.times.10.sup.-10 m and a capacity C.sub.Na of sodium ion take-up, expressed in grams of Na per 100 grams of modified zeolite, neutralised and then calcined, higher than about 0.85 (the capacity C.sub.Na of sodium ion take-up will be defined more precisely in the following paragraph); a specific surface area determined by the B.E.T. method higher than about 400 m.sup.2.g.sup.-1 and preferably higher than 550 m.sup.2 /g, a water vapor adsorption capacity at 25.degree. C. for a partial pressure of 2.6 torrs (346.6 Pa) higher than about 6%, a pore distribution comprising between 1 and 20% and preferably between 3 and 15% of the pore volume contained in pores of diameter situated between 20 and 80 .ANG., (20 and 80.times.10.sup.-10 m) the remainder of the pore volume being contained in pores of diameter less than 20.times.10.sup.-10 m.
The various characteristics are measured by the method specified below:
the molar ratio SiO.sub.2 /Al.sub.2 O.sub.3 is measured by chemical analysis. When the amounts of aluminum become low, for example less than 2%, for more accuracy it is opportune to use a method of determination by atomic adsorption spectometry.
the mesh parameter is calculated from the X-ray diffraction diagram, by the method described by ASTM card D 3. 942-80. It is clear that to carry out this calculation correctly, the crystallinity of the product must be sufficient.
The specific surface area is determined by measurement of the nitrogen absorption isotherm at the temperature of liquid nitrogen and calculated by the conventional B.E.T. method, the specimens are pre-treated, before the measurement, at 500.degree. C. with dry nitrogen flushing.
the pore distribution is determined by the B.J.H. method described by BARRETT, JOYNER and HALENDA in the Journal of the American Chemical Society, volume 73, page 373-1951. This method is based on the numerical exploitation of the nitrogen desorption isotherm. The measurement is carried out with a CARLO EPRA type SORPTOMATIC 1800 Series apparatus. The results are expressed by the values of the porevolume V as a function of the diameter of the pores D; the derivative curve is also presented: dV/dD as a function of D. The total pore volume is defined as the volume of nitrogen absorbed at saturation, more exactly at a partial pressure corresponding to a ratio between the partial pressure and the saturating vapor pressure P/Po equal to 0.99.
the percentages of water take-up (or water vapor adsorption capacity) are determined by means of a conventional gravimetric apparatus. The specimen is pre-treated at 400.degree. C. under a primary vacuum then brought to a stable temperature of 25.degree. C. A water pressure of 346.6 Pa is then admitted, which corresponds to a ratio P/Po of about 0.10 (ratio between the partial pressure of water admitted into the apparatus, and the saturated vapor pressure of water at a temperature of 25.degree. C.).
the exchange capacity of the sodium ions C.sub.Na (or sodium ion take-up capacity) is determined in the following manner: a gram of zeolite is subjected to three successive changes in 100 cm.sup.3 of 0.2 M NaCl solution, for one hour at 20.degree. C. with good stirring. The solutions are left at a natural pH during the exchange. In fact if the pH were readjusted to values close to 7 by the addition of small amounts of soda, the sodium levels exchanged would be higher. It is expressed as grams of sodium per 100 grams of modified zeolite, re-exchanged the calcined at 1100.degree. C.
It has been discovered in the present invention that stabilized Y zeolites corresponding to the aforementioned specifications had remarkable properties;
These zeolites are manufactured, generally from a Y-Na zeolite, by a suitable combination of the two basic treatments: (a) a hydrothermic treatment which associates temperature and partial pressure of water vapor, and (b) an acid treatment, by, a strong and concentrated inorganic acid.
Generally the Y-Na zeolite from which the zeolite according to the invention is prepared possesses a molar ratio SiO.sub.2 /Al.sub.2 O.sub.3 comprised between about 4 and 6; it is convenient at first to lower the sodium content (by weight) thereof to a value of the order of 1 to 3% and preferably to less than 2.5%; Y-Na zeolite moreover generally possesses a specific surface area comprised between 750 and 950 m /g approximately.
Several modifications exist of the preparation which all follow the hydrothermic treatment of the zeolite by an acid treatment. Hydrothermic treatments are operations known in the Prior Art and enable the production of so-called stabilised or again ultra-stabilized zeolites. Thus MACDANIEL and MAYER claimed in U.S. Pat. No. 3,293,192 the production of so-called ultra-stable zeolite Y characterised by a crystalline parameter of 24.45 to 24.2 and low percentages of sodium, due to the association of hydrothermic treatments and cationic exchanges by solutions of ammonium salt, KERR et al. have also obtained Y zeolites enriched in silica by selective extraction of the aluminum by means of a chelating agent such assethyylene diamine tetraacetic acid (U.S. Pat. No. 3,442,795).
EBERLY et al have combined the two latter techniques for the production of dealurinised zeolites (U.S. Pat. Nos. 3,506,400 and 3,591,488). They show that the hydrothermic treatment consists of selectively extracting the tetracoordinated aluminum from the aluminosilicate frame. They claim this procedure as well as the subsequent treatment by solutions containing various cations. One example is given with subsequent extraction by 0.1 N HCl resulting in a faujasite no longer containing aluminum.
WARD describes the manufacture of zeolithic catalysts intended for the manufacture of middle distillates (U.S. Pat. No. 3,853,742). The zeolite is stabilized but it is not subjected to an acid treatment at the end of the series of treatments, and its crystalline parameter is comprised between 24.40.times.10.sup.-10 m and 24.50.times.10.sup.-10 m. BEZMAN and RABO have used as a base hydrocracking catalysts from more highly stabilized zeolites, whose crystalline parameter varies from 24.20.times.10.sup.-10 m to 24.45.times.10.sup.-10 m (EP Pat. No. 0028938). This type of zeolite is more particularly characterised by a ion exchange capacity "IEC" less than 0.07. The exchange capacity is defined in this patent as: ##EQU1##
k being the molar ratio SiO.sub.2 /Al.sub.2 O.sub.3 determined before the retroexchange with the NA+ ions. A zeolite of molar ratio SiO.sub.2 /Al.sub.2 O.sub.3 equal to k and of IEC equal to 0.07 corresponds to the approximate formula: EQU H.sub.0.93 Na.sub.0.07 AlO.sub.2 (SiO.sub.2).sub.k/2
The sodium ion take-up capacity of such a product, expressed in g per 100 g, is: ##EQU2## When k=4.8, C.sub.Na =0.78 When k=10, CNa=0.45
Hence for a value of IEC less than or equal to 0.07, the sodium ion take-up capacity C is in all cases less than 0.8, (k is higher than 4.8 for a stabilized zeolite).
Zeolite ultra-stabilized by the method of BEZMAN and RABO is also characterised by a hydrophobic character, such as its water absorption capacity at 25.degree. C. and a value of P/Po of 0.1 namely less than 5%.
SCHERZER (Journal of Catalysis 54, 285, 1978) synthesised by a combination of hydrothermic and acid treatments, and characterised by X-ray diffraction zeolites very much enriched in silica (molar ratio SiO.sub.2 /Al.sub.2 O.sub.3 100). At the same period, V. BOSACEK et al carried out also similar treatments to obtain an ultra-stable zeolite ratio SiO.sub.2 /Al.sub.2 O.sub.3 of the order of 75.
These products are too highly dealuminised and for this reason, their interest for hydrocracking is doubtful. In fact a minimum of aluminum atoms must be maintained in the structure to keep sufficient acidity in the structure necessary for the hydrocracking catalyst;
Belgian patent No. 896,873 indicates the possibility of effecting hydrocracking to produce middle distillates by means of a catalyst containing Y zeolites treated with water vapor and then lixiviated. This ultra-stabilised zeolite is characterised by different parameters, particularly a molar ratio SiO.sub.2 /Al.sub.2 O.sub.3 higher than 10, a crystalline parameter less than 24.4.times.10.sup.-10 m; and a particular mesopore distribution. The porosity of a Y zeolite, untreated with water vapor and by acid is entirely comprised of pores of diameter less than 20.times.10.sup.-10 m.
The ultra-stabilisation treatments modify this distribution. In this Belgian Patent No. 895,873, the treatments described create a meso pore centred at about 80.times.10.sup.-10 m for a zeolite treated with water vapor and at about 135.times.10.sup.-10 m for the same zeolite subjected subsequently to acid treatment.